Abstract: :
Purpose: Strabismic amblyopes underestimate the number of featurespresented to their amblyopic eye. This undercounting representsa limitation in their ability to distinguish and to individuatethe features with their amblyopic eye. The purpose of this studywas to ask whether the undercounting shown by amblyopes is dueto a limit in attentional capacity (i.e. the number of itemsone can attend to) or a limit in attentional resolution (i.e.in the spacing of items).Methods: Five amblyopes and threenormal observers participated in the study. Viewing was monocular.The stimuli were briefly presented, high contrast (100%) Gaborpatches. In experiment 1, N patches (where N= 3, 5 or 7) werepresented, randomly positioned on a 7X7 square array of possiblepositions. The observers recorded the number of patches presentedin a given trial. In experiment 2, the stimulus was a 7X7 squarearray of Gabor patches, with N patches randomly removed fromthe array (where N = 3, 5 or 7). The observers recorded thenumber of «missing patches’. In experiment 3 observershad to estimate the number of vertical Gabor patches (N= 3,5 or 7) among distractor horizontal Gabor patches in the same7X7 array. A capacity limit predicts that undercounting willincrease with N; a resolution limit predicts that undercountingwill increase as separation decreases.Results: Amblyopic observersmarkedly undercounted the number of features (experiment 1)number of missing features (experiment 2) and number of differentfeatures (experiment 3) with their amblyopic eyes. Undercountingwas maximum when N was greater than 3 and at greater viewingdistances. Undercounting was either independent of separation,or decreased as patch separation decreased, opposite from theresolution prediction.Conclusion: Our data suggests that theundercounting seen in amblyopic observers can be attributedto a limit in capacity rather than to a limit in resolution.Since low-level explanations cannot completely explain the separationindependence we suggest that our results reflect a higher-levellimitation in the number of features the amblyopic observerscan individuate.